If we knew the damage each volley does (infocards display average damage rather than damage done by a volley), it sounds like it would be simple enough to calculate exactly how much damage the target ship should receive in such a situation, turn autofire on for a second to shoot a volley, turn it off again, and compare predicted vs actual results to see which hypothesis seems more accurate.
I highly suspect that the mitigation is increased immediately, but it'd be great to know for certain.
Does anyone know if we have access to that information? (damage per volley, volley rate)